EUROX

iTrader: (15)

Ok, I did a search for coolant and found a great deal of information on my problem, however alot of it is either contradictory at times or vague. I've also looked through Rob Robinette's FAQ and didn't find what I was looking for.

Anyways, I need to know about the caps on the AST and the Filler neck (not the overflow tank):

1) Which of these caps is a flat cap and which one is supposed to be a pressure valve cap (one with a spring)?

2) If I replace the caps, what PSI caps are they supposed to be?

3) Will any cap from Kragen do? Or will only the Mazda OEM parts do?


The reason why I am asking (and I'm sure you have heard this many times before) is that while driving my new FD recently, the temperature gauge pegged itself at H suddenly and steam started to come from the front passenger headlight. I immediately pulled over and ran the fans with the hood open. Coolant started to flow out the bottom of the front passenger side of the car. Looking at it, it appeared as if I blew a hose to the overflow tank. After returning with more coolant, I filled the overflow to more than full, then started the car and limped the last block to my house.

Later after it cooled down, I checked the coolant level in the overflow tank and found it below Low. I added coolant to bring it to Full. I then started the car and idled it for 6 minutes with the heat on, and after a little while, a small amount of coolant leaked from the area near the Overflow tank again. I stopped the engine. The next day, I put the car on jackstands and checked for leaks in the vicinity. I could not find any loose hoses, no obvious splits in the overflow tank. I used electrical tape to wrap the joint between the hose going to the Overflow and the tank itself.

So I refilled the overflow tank to Full, then started the car. No coolant smelling white smoke from the tailpipe and the car idled for 30 minutes without the temp gauge moving above horizontal. I was able to drive the car around town without it overheating and the temp gauge appeared normal. It used to gurgle before after driving, but this time it did not.

After letting it sit overnight, I checked the level in the overflow tank again, and now it is down from Full to Low. People have said that it is supposed to suck coolant from that tank into the engine - is that normal? The coolant levels in the AST and Filler neck appear to be topped off. Should I add more coolant to the Overflow tank to bring it back to Full again?

I am confused since I would think that you check the coolant level from the overflow tank since it is the only coolant point with a dipstick. However, people are saying to check it from the Filler and not the Overflow.

Could you help out a confused new FD owner? Will my FD overheat the next time I take it on a drive or do I just need to burp the system? Will replacing the caps help?

Thanks - I appreciate any help anyone can give.

BrianK

1) Radiator cap has the spring

2) 12 psi (may be 13 - check the workshop manual)

3) Any cap will do - you might want to get a clamping cap for the AST (which I hope is aluminum - if it's not, replace it with an Al one ASAP)

You should fill the cooling system at the filler neck - it should be all the way to the top when the car is cold. The overflow tank is exactly what it's called - a place for coolant to go as opposed to leaking out of the system - it's not an accurate gauge of how much coolant is in your car because the level is dependant on the amount of coolant that has overflowed (which is dependant on temperature and pressure of the system). It is, however, the only way to determine anything about the coolant when the car is hot (because you obviously can't take the radiator cap off when the car is hot) - hence the dipstick.

To make your life easier, disconnect the coolant hose that feeds the throttle body when filling at the filler neck. Keep filling until coolant comes out of that hose. You'll probably have to burp the system one time after you fill it (burping the system means starting it up, letting it idle with the heat on until you feel heat, turning it off, let it sit for a minute, then fill some more).

The most important thing you're going to do right now (after filling to the filler neck) is get your system pressure tested for leaks. There's LOTS of places in the FD cooling system that are notorious for cracking & leaking - electrical tape is not a permanent answer. You'll want to check the radiator end tanks, AST, and coolant lines (especially those going to the turbo) as a bare minimum. Look for any signs of leaking - you may want to wipe everything down so it's dry, go out for a drive, come back & check for wetness. A white glove or rag might make your life easier for the hard to see places like bottom side of the radiator - wipe it & see what comes off on the rag.

Also, make sure when you're doing all of this filling that you're using a proper coolant/water mixture - too much coolant is hard on the water pump. I used 35% coolant for down here in So Cal.

ahh the joys of the FD cooling system. IMHO, all FDs should have Aluminum radiators and ASTs as some of the "must have" reliability mods. I bought my Al AST used for $80 (on the parts trader) and the Al Fluidyne radiator was just over $400 new.

Good luck!

one final edit: the gurgling sound comes from boiling (obviously). Pressure increases the boiling temp. Inversly, the lack of pressure will reduce the increase in the boiling temp. My point: when you hear gurgling you're either REALLY hot or have a leak in the cooling system that's causing it to loose pressure.
Boiling *is* bad, but you don't want to try to avoid it by getting a higher pressure cap - the FD cooling system can't handle the high pressure.

EUROX

iTrader: (15)

Thanks a bunch, BrianK! I really appreciate it!

As a matter of fact, my aluminum AST was on order when this happened. I'm planning on ordering a Fluidyne radiator within the month.

A few questions about your reply - when you say that the radiator cap is the one with the spring, could you help me understand which cap is the radiator one?

(While standing facing the RX7's hood looking at the windshield) The AST is the blocky black tank near the front next to the intercooler? And the radiator filler is the cap that's farther back and closer to being above the turbos? Are both caps supposed to be 12 psi? Or is that only the radiator cap, since the other cap is supposed to be just a flat cap?

Thanks again!

SleepR1

Nice of BrianK to answer considering he's not an FD owner anymore

adam c

iTrader: (2)

Brian K gave you a lot of good information, except there is no radiator cap. There is a cap on the air seperator tank (which you are replacing) and one on the filler neck. The cap on the aftermarket ast should have a relief valve. The filler neck cap has no valve.

As the coolant temperature rises, the coolant takes up more space, and will overflow a little bit in the overflow tank. When the temperature drops, it sucks the coolant back from the overflow tank. There is not a great deal of fluid going back and forth, but it would explain why you filled the tank, it cooled off, and there was less fluid in the tank later.

EUROX

iTrader: (15)

Thanks for the replies and advice, guys.

Here's the status... I got new OEM caps for the Filler neck and AST from Mazda and installed those. Upon looking closely, I realized a problem. The caps on the Filler and AST were reversed! So the AST cap with the spring was on the Filler and vice versa! I switched them and topped off coolant in both locations and then ran the car and haven't gotten any overheating yet. I should clarify that this was just idling for about 40 minutes in the garage with the heat on. No leaks from the Overflow and the Overflow level is at Full again. No white sweet smelling smoke either. I think I avoided a bullet.

Does anyone have any theories on what exactly happens with the coolant system if the caps are reversed?

BrianK

sorry, by "radiator cap" I meant "cap on the filler neck"

Wasn't there still that issue of you seeing coolant coming from under the overflow?

I don't have any theories on the caps being reversed - it should be the same pressure all the way around, but maybe the way the two caps seal is different, so you weren't getting a proper seal.

speaking of not owning an FD anymore - I took the beat up, old '86 Toyota MR2 to the Ferrari/Lamborghini/Lotus/Bently dealer in Beverly Hills today to put my deposit down on the new car... heheh feels kinda weird rolling up to a showroom with $200K cars in my $2K beater (that's been totalled twice).

aReX-7

iTrader: (3)

The AST cap is actually rated at 0.9 bar, or ~13 psi. The two caps (AST and filler neck) function differently, and switching them explains why your car boiled over. Here is the concept.

The filler neck cap (w/o spring) simply seals off the filler neck. Near the top of the filler neck, there is a hose that goes over to the AST that is completely unrestricted, therefore keeping the filler neck and the AST at a common pressure. The cap on the AST has a spring loaded seal in it that keeps coolant from flowing from the AST into the overflow tank until system pressure exceeds 13 psi. If the caps are switched, coolant will flow freely from the AST to the overflow tank with little or no system pressure.

The reason your car overheated (boiled the coolant) is because the the boiling point of the water/antifreeze mixture is only ~100 degees C at atmospheric pressure, and the fans do not come on until 110 degrees C with the fans and park lights off. With the caps in the correct position, the system will build pressure up to 13 psi and therefore raise the boiling point of the coolant. This prevents boiling of the coolant. Of course, when your car cools off, the spring cap on the AST allows coolant to be sucked back into the system from the overflow tank.

So, your coolant didn't necessarily get too hot, it just boiled over because system pressure was zero. The needle went up because it was measureing steam. Don't get me wrong, this is not good for the engine, but at least it sounds like you didn't heat the sucker up to 125 deg C.

Sounds like you should be fine, but keep an eye on things. Thermostats go bad in these cars ALOT. I have changed mine as many as 3 times in 12 months! An aftermarket temp gauge can help tremendously in diagnosing a cooling problem before it is too late. Just a word to the wise.

Good luck,

Jeff

BrianK

ahh... a difference in opinion as far as which cap is which... I *thought* that the filler neck cap had a spring and the AST did not, but I can't check 'cause I don't have the car anymore.

Here's a good way to check, my AST cap had a sticker on it that said something to the effect of "Do not open, ever"

It's been so long since I fooled with the coolant on a 7 that I may have them switched in my head.

adam c

iTrader: (2)

Not an "opinion". The spring cap with the release valve goes on the AST. That's a "fact"...............Jack??

n2trbl

It's the same for me. I haven't repalced my ast yet and there is no spring on the cap and it does in fact say "do not open ever" on it too. I do have one question though. How much coolant is there supposed to be in the ast? I thought a little bit was normal but topping it off? I've never heard anyone talk about topping off their ast. Please clue me in on this matter. Thanks!

aReX-7

iTrader: (3)

If the filler neck is topped off, the AST will be full. Remember, there is a hose that runs fron the filler neck to the AST.

Hey Brian, I guess I could go check one of my cars again. I have three FD's right now. Well, one of them is sold, but it won't be gone until next week.

EUROX

iTrader: (15)

Actually, after checking the levels on all 3 points including the overflow, I'm at full on all and I haven't leaked from the overflow since the first time. I assume it may have been because I was overfilling the overflow and not adding any coolant to the Filler neck. I hope this problem is solved, but just to make sure I am going to install the aluminum AST, a new overflow tank, a new hose from the AST to the overflow tank, and a Fluidyne radiator.

I still have to take the car on an extended drive though to make sure.

EUROX

iTrader: (15)

Ok, so to doublecheck if the caps were indeed in the right places, I not only checked the diagram that the Mazda parts guy was looking at and got a good look. The totally round cap has a spring and indeed does go onto the AST. The round cap with bumps on either side does not have a spring and goes on the Filler neck.

I also looked at the Mazda RX7 Work Manual (big green thick book) and the diagram on the coolant system section also shows the caps arranged as such.

To triplecheck with a decidedly less reliable source, I merely opened the issue of Super Street I have with a green FD built by RE Amemiya (streetable version - not the track one) and similarly saw the caps in the same place. Funny way to check though...

Now here's another question. When I was looking through Stant's catalog for coolant caps, I noticed the 93 FD is set to have a 13 psi cap, while the 94 is set to have a 16 psi cap. Should I use the 16 psi cap to better cool my system?